Darmanitan is one of my favourite pokemon in 5th gen because it looks Reckless but i never try it so far.
Due to the fact that It can change to Zen mode when its HP is lower than 50% but i wanna ask that after it changes to Zen mode and battle ends, then is it still in Zen mode or it will change back to normal form.
Please tell me and recommend if i should train it or not?

Well, Zen Mode is an ability that occurs when at low HP. If you have low Health and go into battle in the wild, then Darmanitan will transform into Zen Mode Darmanitan. But if you have full health then it will half to drop its HP to less than half for it to activate. I recommend Zen Mode Darmanitan if you plan to use a strategy with it. If not, then stick with Sheer Force to have an amazing sweeper in your arsenal!